I have a MBR-formatted, bootable USB flash drive with GRUB. I have tested it successfully on other computers - one with BIOS and another one with UEFI.
I usually boot my 2011 4,1 MacBookAir with that flash drive. The last time I did that was 21 October.
Yesterday, I restarted, pressed Option after the gray screen as usual, but I only saw "Macintosh HD" and "Recovery" - I do not see the flash drive, which is typically labeled "Windows".
I zeroed the key and reinstalled the bootloader, but that did not fix the issue. However, when I connect a GPT-formatted disk, the boot screen shows it with the label "Windows", as it normally does.
It would be helpful to know whether other people can (or cannot) reproduce the issue on the same model.